var canvas;
var gl;

var cubeVerticesBuffer;
var cubeVerticesTextureCoordBuffer;
var cubeVerticesIndexBuffer;
var cubeVerticesIndexBuffer;
var cubeRotation = 0.0;
var lastCubeUpdateTime = 0;

var cubeImage;
var cubeTexture;

var mvMatrix;
var shaderProgram;
var vertexPositionAttribute;
var vertexNormalAttribute;
var textureCoordAttribute;
var perspectiveMatrix;

//
// start
//
// Called when the canvas is created to get the ball rolling.
//
function start() {
    canvas = document.getElementById("glcanvas");

    initWebGL(canvas);      // Initialize the GL context

    // Only continue if WebGL is available and working

    if (gl) {
        gl.clearColor(0.0, 0.0, 0.0, 1.0);  // Clear to black, fully opaque
        gl.clearDepth(1.0);                 // Clear everything
        gl.enable(gl.DEPTH_TEST);           // Enable depth testing
        gl.depthFunc(gl.LEQUAL);            // Near things obscure far things

        // Initialize the shaders; this is where all the lighting for the
        // vertices and so forth is established.

        initShaders();

        // Here's where we call the routine that builds all the objects
        // we'll be drawing.

        initBuffers();

        // Next, load and set up the textures we'll be using.

        initTextures();

        // Set up to draw the scene periodically.

        setInterval(drawScene, 15);
    }
}

//
// initWebGL
//
// Initialize WebGL, returning the GL context or null if
// WebGL isn't available or could not be initialized.
//
function initWebGL() {
    gl = null;

    try {
        gl = canvas.getContext("experimental-webgl");
    }
    catch(e) {
    }

    // If we don't have a GL context, give up now

    if (!gl) {
        alert("Unable to initialize WebGL. Your browser may not support it.");
    }
}

//
// initBuffers
//
// Initialize the buffers we'll need. For this demo, we just have
// one object -- a simple two-dimensional cube.
//
function initBuffers() {

    // Create a buffer for the cube's vertices.

    cubeVerticesBuffer = gl.createBuffer();

    // Select the cubeVerticesBuffer as the one to apply vertex
    // operations to from here out.

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesBuffer);

    // Now create an array of vertices for the cube.

    var vertices = [
        // Front face
        -1.0, -1.0,  1.0,
        1.0, -1.0,  1.0,
        1.0,  1.0,  1.0,
        -1.0,  1.0,  1.0,

        // Back face
        -1.0, -1.0, -1.0,
        -1.0,  1.0, -1.0,
        1.0,  1.0, -1.0,
        1.0, -1.0, -1.0,

        // Top face
        -1.0,  1.0, -1.0,
        -1.0,  1.0,  1.0,
        1.0,  1.0,  1.0,
        1.0,  1.0, -1.0,

        // Bottom face
        -1.0, -1.0, -1.0,
        1.0, -1.0, -1.0,
        1.0, -1.0,  1.0,
        -1.0, -1.0,  1.0,

        // Right face
        1.0, -1.0, -1.0,
        1.0,  1.0, -1.0,
        1.0,  1.0,  1.0,
        1.0, -1.0,  1.0,

        // Left face
        -1.0, -1.0, -1.0,
        -1.0, -1.0,  1.0,
        -1.0,  1.0,  1.0,
        -1.0,  1.0, -1.0
    ];

    // Now pass the list of vertices into WebGL to build the shape. We
    // do this by creating a Float32Array from the JavaScript array,
    // then use it to fill the current vertex buffer.

    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);

    // Set up the normals for the vertices, so that we can compute lighting.

    cubeVerticesNormalBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesNormalBuffer);

    var vertexNormals = [
        // Front
        0.0,  0.0,  1.0,
        0.0,  0.0,  1.0,
        0.0,  0.0,  1.0,
        0.0,  0.0,  1.0,

        // Back
        0.0,  0.0, -1.0,
        0.0,  0.0, -1.0,
        0.0,  0.0, -1.0,
        0.0,  0.0, -1.0,

        // Top
        0.0,  1.0,  0.0,
        0.0,  1.0,  0.0,
        0.0,  1.0,  0.0,
        0.0,  1.0,  0.0,

        // Bottom
        0.0, -1.0,  0.0,
        0.0, -1.0,  0.0,
        0.0, -1.0,  0.0,
        0.0, -1.0,  0.0,

        // Right
        1.0,  0.0,  0.0,
        1.0,  0.0,  0.0,
        1.0,  0.0,  0.0,
        1.0,  0.0,  0.0,

        // Left
        -1.0,  0.0,  0.0,
        -1.0,  0.0,  0.0,
        -1.0,  0.0,  0.0,
        -1.0,  0.0,  0.0
    ];

    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertexNormals),
        gl.STATIC_DRAW);

    // Map the texture onto the cube's faces.

    cubeVerticesTextureCoordBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesTextureCoordBuffer);

    var textureCoordinates = [
        // Front
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0,
        // Back
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0,
        // Top
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0,
        // Bottom
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0,
        // Right
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0,
        // Left
        0.0,  0.0,
        1.0,  0.0,
        1.0,  1.0,
        0.0,  1.0
    ];

    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(textureCoordinates),
        gl.STATIC_DRAW);

    // Build the element array buffer; this specifies the indices
    // into the vertex array for each face's vertices.

    cubeVerticesIndexBuffer = gl.createBuffer();
    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVerticesIndexBuffer);

    // This array defines each face as two triangles, using the
    // indices into the vertex array to specify each triangle's
    // position.

    var cubeVertexIndices = [
        0,  1,  2,      0,  2,  3,    // front
        4,  5,  6,      4,  6,  7,    // back
        8,  9,  10,     8,  10, 11,   // top
        12, 13, 14,     12, 14, 15,   // bottom
        16, 17, 18,     16, 18, 19,   // right
        20, 21, 22,     20, 22, 23    // left
    ]

    // Now send the element array to GL

    gl.bufferData(gl.ELEMENT_ARRAY_BUFFER,
        new Uint16Array(cubeVertexIndices), gl.STATIC_DRAW);
}

//
// initTextures
//
// Initialize the textures we'll be using, then initiate a load of
// the texture images. The handleTextureLoaded() callback will finish
// the job; it gets called each time a texture finishes loading.
//
function initTextures() {
    cubeTexture = gl.createTexture();
    cubeImage = new Image();
    cubeImage.crossOrigin = "";
    cubeImage.onload = function() { handleTextureLoaded(cubeImage, cubeTexture); }
    cubeImage.src = "textures/floor.png";
}

function handleTextureLoaded(image, texture) {
    gl.bindTexture(gl.TEXTURE_2D, texture);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR_MIPMAP_NEAREST);
    gl.generateMipmap(gl.TEXTURE_2D);
    gl.bindTexture(gl.TEXTURE_2D, null);
}

//
// drawScene
//
// Draw the scene.
//
function drawScene() {
    // Clear the canvas before we start drawing on it.

    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

    // Establish the perspective with which we want to view the
    // scene. Our field of view is 45 degrees, with a width/height
    // ratio of 640:480, and we only want to see objects between 0.1 units
    // and 100 units away from the camera.

    perspectiveMatrix = makePerspective(45, 640.0/480.0, 0.1, 100.0);

    // Set the drawing position to the "identity" point, which is
    // the center of the scene.

    loadIdentity();

    // Now move the drawing position a bit to where we want to start
    // drawing the cube.

    mvTranslate([0.0, 0.0, -6.0]);

    // Save the current matrix, then rotate before we draw.

    mvPushMatrix();
    mvRotate(cubeRotation, [1, 0, 1]);

    // Draw the cube by binding the array buffer to the cube's vertices
    // array, setting attributes, and pushing it to GL.

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesBuffer);
    gl.vertexAttribPointer(vertexPositionAttribute, 3, gl.FLOAT, false, 0, 0);

    // Set the texture coordinates attribute for the vertices.

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesTextureCoordBuffer);
    gl.vertexAttribPointer(textureCoordAttribute, 2, gl.FLOAT, false, 0, 0);

    // Bind the normals buffer to the shader attribute.

    gl.bindBuffer(gl.ARRAY_BUFFER, cubeVerticesNormalBuffer);
    gl.vertexAttribPointer(vertexNormalAttribute, 3, gl.FLOAT, false, 0, 0);

    // Specify the texture to map onto the faces.

    gl.activeTexture(gl.TEXTURE0);
    gl.bindTexture(gl.TEXTURE_2D, cubeTexture);
    gl.uniform1i(gl.getUniformLocation(shaderProgram, "uSampler"), 0);

    // Draw the cube.

    gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, cubeVerticesIndexBuffer);
    setMatrixUniforms();
    gl.drawElements(gl.TRIANGLES, 36, gl.UNSIGNED_SHORT, 0);

    // Restore the original matrix

    mvPopMatrix();

    // Update the rotation for the next draw, if it's time to do so.

    var currentTime = (new Date).getTime();
    if (lastCubeUpdateTime) {
        var delta = currentTime - lastCubeUpdateTime;

        cubeRotation += (30 * delta) / 1000.0;
    }

    lastCubeUpdateTime = currentTime;
}

//
// initShaders
//
// Initialize the shaders, so WebGL knows how to light our scene.
//
function initShaders() {
    var fragmentShader = getShader(gl, "shader-fs");
    var vertexShader = getShader(gl, "shader-vs");

    // Create the shader program

    shaderProgram = gl.createProgram();
    gl.attachShader(shaderProgram, vertexShader);
    gl.attachShader(shaderProgram, fragmentShader);
    gl.linkProgram(shaderProgram);

    // If creating the shader program failed, alert

    if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
        alert("Unable to initialize the shader program.");
    }

    gl.useProgram(shaderProgram);

    vertexPositionAttribute = gl.getAttribLocation(shaderProgram, "aVertexPosition");
    gl.enableVertexAttribArray(vertexPositionAttribute);

    textureCoordAttribute = gl.getAttribLocation(shaderProgram, "aTextureCoord");
    gl.enableVertexAttribArray(textureCoordAttribute);

    vertexNormalAttribute = gl.getAttribLocation(shaderProgram, "aVertexNormal");
    gl.enableVertexAttribArray(vertexNormalAttribute);
}

//
// getShader
//
// Loads a shader program by scouring the current document,
// looking for a script with the specified ID.
//
function getShader(gl, id) {
    var shaderScript = document.getElementById(id);

    // Didn't find an element with the specified ID; abort.

    if (!shaderScript) {
        return null;
    }

    // Walk through the source element's children, building the
    // shader source string.

    var theSource = "";
    var currentChild = shaderScript.firstChild;

    while(currentChild) {
        if (currentChild.nodeType == 3) {
            theSource += currentChild.textContent;
        }

        currentChild = currentChild.nextSibling;
    }

    // Now figure out what type of shader script we have,
    // based on its MIME type.

    var shader;

    if (shaderScript.type == "x-shader/x-fragment") {
        shader = gl.createShader(gl.FRAGMENT_SHADER);
    } else if (shaderScript.type == "x-shader/x-vertex") {
        shader = gl.createShader(gl.VERTEX_SHADER);
    } else {
        return null;  // Unknown shader type
    }

    // Send the source to the shader object

    gl.shaderSource(shader, theSource);

    // Compile the shader program

    gl.compileShader(shader);

    // See if it compiled successfully

    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        alert("An error occurred compiling the shaders: " + gl.getShaderInfoLog(shader));
        return null;
    }

    return shader;
}

//
// Matrix utility functions
//

function loadIdentity() {
    mvMatrix = Matrix.I(4);
}

function multMatrix(m) {
    mvMatrix = mvMatrix.x(m);
}

function mvTranslate(v) {
    multMatrix(Matrix.Translation($V([v[0], v[1], v[2]])).ensure4x4());
}

function setMatrixUniforms() {
    var pUniform = gl.getUniformLocation(shaderProgram, "uPMatrix");
    gl.uniformMatrix4fv(pUniform, false, new Float32Array(perspectiveMatrix.flatten()));

    var mvUniform = gl.getUniformLocation(shaderProgram, "uMVMatrix");
    gl.uniformMatrix4fv(mvUniform, false, new Float32Array(mvMatrix.flatten()));

    var normalMatrix = mvMatrix.inverse();
    normalMatrix = normalMatrix.transpose();
    var nUniform = gl.getUniformLocation(shaderProgram, "uNormalMatrix");
    gl.uniformMatrix4fv(nUniform, false, new Float32Array(normalMatrix.flatten()));
}

var mvMatrixStack = [];

function mvPushMatrix(m) {
    if (m) {
        mvMatrixStack.push(m.dup());
        mvMatrix = m.dup();
    } else {
        mvMatrixStack.push(mvMatrix.dup());
    }
}

function mvPopMatrix() {
    if (!mvMatrixStack.length) {
        throw("Can't pop from an empty matrix stack.");
    }

    mvMatrix = mvMatrixStack.pop();
    return mvMatrix;
}

function mvRotate(angle, v) {
    var inRadians = angle * Math.PI / 180.0;

    var m = Matrix.Rotation(inRadians, $V([v[0], v[1], v[2]])).ensure4x4();
    multMatrix(m);
}